Constant current electric generator



Sept. 27, 1932. G. AUSTIN ET AL 1,879,014

CONSTANT CURRENT ELECTRIC GENERATOR Filed Jan. 24, 1931 4 Sheets-Sheet l Sept. 27, 1932. G. AUSTIN ET AL 1,879,014

CONSTANT CURRENT ELECTRIC GENERATOR Filed Jan. 24. 1931 4 SheetsSheet 2 Sept. 27, 1932. G AUSTIN ET AL 1,879,014

CONSTANT CURRENT ELECTRIC GENERATOR Filed Jan. 24, 1951 4 Sheets-Sheet Zv Sept. 27, 1932.

G. AUSTINET AL 1,879,014

CONSTANT CURRENT ELECTRIC GENERATOR 4 Sheets-Sheet 4 Filed Jan. 24, 1931 Patented 27, 1932 UNITED STATES P ATENT OFFICE GILBERT aus'rm, um oormmorm norm AND wnmrut ALLAN morum, or enasoow, aco'rmn CONSTANT CURRENT ELECTRIC GENERATOR Application Med January 24, 1981, Herlal No. 510,828, and in Great Britain February 18, 1880.

This invention relates to that type of constant current electric generating p ant wherein the current has an approximatel constant value for all values of the load, t e voltage automatically varyin to suit the load, but in which .the value of t e constant current can be adjusted to any other approximatel constant value lower than its maximum va ue, or may be reversed, and this invention is particularly applicable to ship propulsion systems and the like where lower values of the constant current ma be required for cruising at reduced spec s, or where the current may re uire to be reversed for manmuvermg.

The c ief object of the invention is to pro- ,vide an electric generator of the aforesald type which will have a short circuit current of reasonable size. Another object is to provide an electric generator of the aforesaid type which can be used relative to the propulsion of ships, and in which, if the load is suddenly thrown off or suddenly increased, for exam le, by the propeller leaving the water or ecoming fouled, respectively, the

current will fall very low, or the voltage will fall very low, respectively.

In our invention, we make use of a hitherto little known effect obtainable in specially designed dynamos having under each main pole 0 an air gap which is variable in radial length along the face of the pole in a circumferential direction.

The use of such graded air gaps is already well known in connection with the improvement of the commutation and compounding propertiesof a constant voltage dynamoor motor.

It has already been proposed to utilize armature reaction with a graded air gap for obtaining a slight rise in voltage with increase of load by causing the polar flux on no-load to saturate the leading tip and the corresponding armature teeth, but leaving the trailing tip relatively unsaturated, and causing the armature reaction on load to increase the polar flux on the trailing tip in greater proportion that it can reduce 1t in the leading. tip owing to the effect of this saturation.

In order that our invention may be proper- 1y understood, we show, by way of example on the accompanying drawings, a series 0 explanatory figures wherein Fig. 1 represents in diagrammatic form, one pole of an electric generator or d amo according to our invention with gra ed air gap, but developed out flat for convenience of explanation, this pole being shown with full exciting magneto motive force (M. M. F.) thereon. w

Fig. 2 represents a M. M. F. and flux distribution diagram of the fully excited pole of Fig. 1, showing the effect of armature reaction on the flux distribution.

Fig. 3 represents the same pole as in Fig. 1 but without excitation. Fig. 4 is the M. M. F. and flux distribution iagram corresponding to Fig. 3.

Fig. 5 shows one form of the windings and connections of a dynamo according to our my invention.

Fig. 6 shows a comparison between the characteristic obtainable with a dynamo in accordance with our present invention and that obtainable with a drooping characteristic'dynamo of the well-known 3 coil type.

Fig. 7 shows a oup of characteristic curves obtainable Wltll a dynamo made according to our invention with different values of the exciting current, and when the exciting current is reversed.

Fig. 8 shows our invention applied to a scheme for marine propulsion.

Figs. 9, 10 and 11 show modified forms of the graded air gap and methods of increasing the magnetic reluctance of the pole horns if desired.

Fig. 12 is a diagram illustratin the arrangement of two generators eac driven from a separate prime mover and supplying go a separate motor, switching means being provided for cutting out one of the generators and prime movers and connecting the two motors in series with the remaining generator.

Fig. 13 is a detail diagram illustrating how, with apparatus similar to that shown in Fig. 12, one of the generators may be out out and the two motors may be connected in parallel with the remaining generator.

Reverting to Fig. 1, W is one pole of a machine according to our invention having a aded air gap varying from a minimum ength C E at the ole ti C to maximum length D F at the po e tip X is the armature rotating (in a dynamo) in the direction shown by the arrow r. A and B are the brushes of the machine. The arrow a, shows the direction of the main M. M. F. due to windin s on the ole, and arrows y and 2 represent t e directlon of the M. M. F.s due to the armature current at the pole tips C and D respectively.

Fig. 2 shows a diagram of the M. M. F.s O

alon the pole face and the flux distribution due t ereto. In Fig. 2 the rectangle N Q R 0 shows the M. M. F. due to the pole winding having the value all over the pole face equal to the height N Q. G H J K L is the armature M. M. F. due to the constant current therein. This M. M. F. has the negative value M G at the brush A and the positive value P L at the brush B. The values of the armature M. M. F. at the pole tips C and are respectively N H negative and O K positive. The resultant M. M. F. over the whole pole face is therefore given by the Fig. N S 0 wherein N S is the resultant positive M. M. F. at the tip 0, and O T is the resultant positive M. M. F. at the tip D. If the air gap is so graded that the proportion of the length C E to the length D F is the same as the proportion of N S to O T, then the flux passing across the air gap (neglecting any effect due to saturation of the iron paths) will be represented by the shaded rectangle N U V O, of constant height N U. This is the load condition wherein maximum voltage is generated, the armature current being assumed to be of the constant value.

Should the exciting M. M. F. N Q be reduced in value, the corresponding M. M. F.s N S and O T will be correspondingly reduced and the resultant M. M. F. at the tip C will be reduced to zero and finally reversed reaching the limiting value as shown by Figs. 3 and 4 when the excitation on the main pole has been reduced to zero.

Fig. 3 is Fig. 1 repeated except that. the M. M. F. shown by the arrow at: is reduced to zero and so disappears.

In Fig. at as before G H J K L represents the armature M. M. F. due to the constant current, and as there is no main M. M. F. on the pole, it also represents the total M. M. F. acting on the air gap. It has the negative value N H at the pole tip C and the positive value 0 K at the pole tip D. Since the air gap under G is smaller than under D the resulting negative flux N U at C (neglecting iron saturation) will be much greater than the resulting positive flux 0 V at D. The flux will in fact (neglecting saturation) follow the curve U J V wherein it is seen that the area N U J is much greater than the area 0 V J. This means that under these conditions there is a resultant negative flux in the pole W (Fig. 3) which would, if this condition prevailed, generate an E. M. F. tending to oppose the constant armature current, or in other words, the machine would now be motoring instead of generating. It is obvious that any tendency for the armature current to change, with a 'ven M. M. F. on the field, will result in a c imge of flux tending to prevent the change of current. Thus if the current in Fi 2 should be reduced, the values of the M. F.s N H negative and K positive, would also be reduced with the result that the area of the flux diagram N U V 0 would be increased, thereby increasing the voltage of the machine.

Theabove description, which neglects the effect of saturation in the armature teeth and pole tips is intended merel to show the man ner in which with a grade air gap the armature current controls the flux assing into the armature and therefore the voitage generated.

We have found that a suitable characteristic can be obtained by supplying the field with excitation from a separate source of constant voltage and supplementing this with a shunt winding connected across the brushes of the constant current machine itself. The polar M. M. F. N Q (Fig. 2) would therefore be made up of two parts, one part due to the separate constant voltage supply and the other due to the shunt coil connected across the brushes A B (Fig. 1). That is, the shunt winding and the separately excited winding act together in the same direction to produce the M. M. F. N Q, (Fig. 2). The machine, with this type of winding behaves in a somewhat similar manner to the wellknown 3 coil type of drooping characteristic dynamo wherein the poles are provided with 3 coils, one separately excited from a constant voltage source, one connected in shunt across its own brushes and assisting the separately excited coil, and one connected in series with the armature and opposing the first two coils. made according to our invention we retain the separately excited coil and the shunt coil but eliminate the opposin series coil of the 3 coil dynamo, its place eing taken by the armature reaction acting across the graded air gap as described above.

This results in very important advantages being obtained in a machine made according to our invention as compared with the wellknown 3 coil dynamo or other well-known drooping characteristic machines, in respect that not only is the weight and cost of the third series winding eliminated but the space saved enables a smaller and cheaper machine to be designed for the same duty and it has the further important advantage that, as will be described later, the characteristic curve obtainable is better than that obtainable from In one form of the machine the 3 coil type of machine, in that for the same working voltage it does not require such a high open circuit voltage, and the material is in conse uence better utilized. Still further, the c aracteristic is such that in the ship propulsion scheme to be described later, there is no danger of the propeller racing if it should leave the water.

Fig. shows in diagrammatic form a dynamo made according to our invention wherev in A is the armature revolving in the direcare the constant current leads.

tion of the arrow. P1, P2, are the shoes of the field poles with the graded air gaps, the smaller air gap being at the side towards which a point on the armature surface approaches, B1, B2 are the brushes, E1, E2 t e separately excited coils actin in the direction of the arrow :0 and supplied by the constant voltage exciter D. S1, S2 are the shunt coils connectedacross the brushes B1, B2 and acting ma netically in the same direction was the coils 1,E2. Aregulating resistance R1 is connected in the circuit supplying E1, E2, and if desired, another regulating resistance R2 can be connected in the shunt circuit S1, S2 and used for varying the shape of the characteristic curve obtained. The direction of the armature M. M. F. is indicated by the dotted lines 3 1, 3 2 with arrow heads, being such that it assists the main M. M. F. over the larger side of the air gap and opposes the main M. F. over the smaller side of the air gap-L1, L2

poles J1, J2 may be used not only for their normal function of assisting commutation but also for varying the characteristic of the machine by shifting the brushes within the limit of movement allowed by the width of the interpole face. Also they may be used to improve the stability of the machine by eliminating short circuit currents due to the armature cross flux acting on the short circuited coil undergoing commutation. In Fig.

6 the heavy line b2, a2, (12 shows the usual type of characteristic curve obtainable from a dynamo of the 3 coil type, and the dotted line b1, a1, d1 shows the type of curve to be expected in a dynamo according to our invention. It will be seen that for a given initial voltage b2 the working voltage a2 of the 3 coil dynamo is very low, and although the shape of the characteristic may be varied in various ways it always suffers from the defect of high open circuit voltage as compared with working voltage.

In a machine under our invention the working voltage a1 is very low below the open circuit voltage 61, and indeed with proper design may be made higher than 61, and this feature has the important advantage that if full load is represented by the current 01 and voltage (11, complete removal of the load (as in the case of a propeller leaving thewater) results in the current falling to zero and Interthe unloaded motor will continue to run at the speed represented by the voltage b. The short circuit current 0 (point d1 on Fi 6) is determined by the value of the M. M. set up by the separately excited coils E1 E2 1g. 5), in conjunction of course with the other constants of the machine, the condition being that the separate excitation, on short circuit should overbalance the reverse M. M. F. set up by the armature (as described with reference to F ig. 4) by an amount just sufficient to generate the voltage necessary to keep the short-circuit current circulatin against the resistances of the armature an external circuit.

If the separate and shunt excitations are reduced by means of the regulators RI and R2 (Fig. 5) a series of characteristic curves can be obtained as shown on the right hand side of the diagram (Fig. 7), and'further, if the separate excitation is reversed, the voltage at the brushes reverses, and consequently also the shunt'excitation, and the armature current. The result is that a series of similar curves is obtained, reversed, however, as to polarity, with the same direction of rotation,

y operation of the regulators R1 and R2, all as shown on the left hand side of Fig. 7.

Although in Fig. 7 we have shown a series of curves of a shape usually desirable in a machine of this type, we may for any special urpose modify the shape/of the curves greaty by altering the amount and relative proportions of the shunt and separately excited winding, by altering the shape of the pole to give the curves shown on Fig. 7, by adding series excitation coils, carrying the whole or a portion of the constant current, in combination with both shunt, and separately excited coils and acting in the same direction, or by combining any or all of the above methods in order to get the exact shape of characteristic desired.

If the regulator R2 is used beyond the value necessary to get the curve b1 (11 d1 (Fig. 6), that is, with a higher resistance, the result will be that the stra ght part of the curve a1 all will tend to slope inwards in some degree, so that the load current C1 will become less than the short circuit current G, and conversely, if R2 has a lower resistance than is necessary to obtain the straight part of the curve a1 (11 it will tend to slope outwards,,that is the load current C1 will be greater than the short circuit current C, and these effects will be more pronounced as the separate excitation is reduced in order to reduce the short circuit current C.

Where series windings are used in conjunction with the other windings as specified above for the purpose of varying the characteristic curve, it will generally be desirable to add a certain amount of saturation at the pole tips in order to stabilize the machine and thus saturation may, as explained above, be added at either or both pole tips.

Fig. 8 shows 01111I1V8I1t101l applied to a system of ship propulsion wherein G is the drooping characteristic dynamo already described. The dynamo G is driven by the main prime mover P which may be a steam or Diesel engine or of any other known type. D is the constant voltage exciter which can be driven directly by the main engine or b an auxiliar engine or in any other suitable manner. is the main propulsion motor, the armature H of which is connected to the propeller I. The field of the motor M is excited by the constant voltage exciter D. The armature H is connected across the constant current circuit L1, L2. A regulator R4 may be connected in the field of M if desired. Regulators R2 and R3 can be used in the shunt field of the generator and exciter if desired. The separatel excited coils E1, E2 of the main generator (3 are excited by the constant voltage dynamo D and include in their circuit the potentiometer type regulator R1 by means of which the excitation of the coils E1, E2 can be reduced and if necessary reversed. The regulator R1 can be located on the bridge of the vessel thereby putting the whole control of the manoeuvrin of the ship directly in the hands of the o cer in charge and avoidingall the delays andrisks of a signalling system. The motor M would normally be operated on full load at or about the point al on the characteristic curve, Fig. 6, and as already explained if the propeller should leave the water, the current will fall to zero with voltage 61 and with full field on the motor the speed would be that corresponding to the voltage 61. Further, should the propeller become fouled by any obstruction or by picking up a rope or cable, thereby bringing the armature H of motor M to stand still, the voltage will fall to zero (point (11 on Fig. 6) with current C, but will not deliver more than its full torque as determined by the current C and the field F1, F2 of the motor. No damage will result to either the generator or the motor as a result of such stoppage of the propeller, and further, in the event of its being necessary to unwind the fouling rope or cable, the motor can be run very slowly in the reverse direction :lndel control of the bridge potentiometer Under the construction provided by the present invention, the flux from each pole neglecting saturation altogether, is given by the formula Where W=axial width of the pole.

L is the length measured circumferentially.

G is the radial length of the airgap at the large end.

' g is the radial length of the airgap at the small end.

M is the armature reaction at the tip of the pole.

X and X are positive M. M. F.s due to pole windings.

The first term of the formula is due to armature reaction, the second term to separate excitation and the third term to the shunt excitation.

The part 1) of the above formula which is due to only, is always negative, the parts (2) and (3) are always positive-if X and X do vnot exist then a negative flux passes through the pole due to the action of the armature M. M. F. on the taper airgap. If G=g that is, a parallel airgap, no reverse flux can pass because (1) =zero.

The system is also applicable to twin screw vessels in which two engines drive two generators G with either two exciters or one common exciter D and two motors M. the event of a breakdown of one generator or its correspondin engine the motors M can be run at reduce speed off one generator, either in series sharing the voltage between them or in parallel sharing the constant current between them as may be most convenient.

In Fig. 12 are shown twin propellers Q, and Q driven respectively by motors M and M supplied with current from separate generators G and G driven respectively by prime movers P and P D is a common exciter for the machines G M G M and is driven by prime mover P Regulators R and. R113, RzA and R23, R3, R'4A and R43 have the same significance as the similar regulators shown in Fig. 8. Should either of the generators G and G or prime movers P P go out of order, or for any other reason, the motors M and M may both be switched into series with the remainin generator, for example, G by means of the series connecting switch V and selector switches W WE, this state being indicated on Fig. 12. Switch W first bridges contacts 14, 15 and short circuits generator G and then, after the field of G is reduced to zero by the potentiometer regulator R the switch W is moved from contact let to contact 15 thereby open, circuiting G and connecting M in series with M Should it be desired to cut out generator G then switch W is retained on contact 14, while switch W is moved from contact 20 to contact 21. When both generators are to be used and the two motors are operated separately, switch V is moved to a position at 90 to that shown, thereby joining contacts 16-17 and 18-19. L L and L L are the main conductors leading from generators G and G to the respective motors.

In Fi 13, in order to switch the motors M and il into arallel and to cut out generator P paralle in switch X is closed, and switch W is move from contact 14, over contact 15 and finally to dead contact 15 thereby open-circuiting generator G If generator G is to be cut out, then switch W is left on contact 14 and switch W is moved graduall to dead contact 21.

The c Dies of series or parallel working will depend on theresults desired. and on the torqueslip characteristics of the propellers Q; and Q More than two motors may of course be use and the connections for these will be obvious to one skilled in the art. I

Although exciter D is shown as driven by a separate prime mover P in Fig. 12, it may be driven by either of the prime movers P or P or, alternatively, two sets identical I with that shown in Fig. 8 may be employed,

each exciter being made large enough to supply two motors, in which case t e motor fields are connected in parallel across the armature of the exciter actually in use.

Although we have described the generator particularly in connection with marine propulsion systems, we do not limit the application of our invention to this, as it obvious that it can be applied to other uses such, for instance, as the propulsion of railway trains.

It is also obvious that modifications will occur to the practical engineer, for instance the exciter D'shown in Fig. 8 may be compound wound and may be made large enough to supplythe ships lighting system, fan and auxiliary motors, etc. in addition to acting as an exciter for the main generator and motor. Further, if a separate source of constant voltage is available, such as a public supply, the exciter D may be eliminated and the coils E1, E2 can be excited from such a separate source.

Withregard tothe grading of the air gap, we do not limit ourselves to the uniformly graded air gap shown in Fi 1 but may for special reasons alter the gra ing in any suitable Way, for instance, as indicated in Figs. 9, 10 and 11, which for convenience of comparison with Fig. 1 are developed out flat.

Further, althou h the description given with reference to igs. 1, 2, 3 and 4 assumed that magnetic saturation was absent, in practice it plays an important part in settling the shape of the characteristic articularly with regard to the maximum vo ta es obtainable, and we may make use of this act to modify enerators and I the shape of the characteristic by increasing the magnetic reluctance of the pole horns by drilling holes as at A, Fig. 9, or nicking the horn as at B' Fig. .10 or by reducing its section as at C ig. 11, or in an other suitable manner, and this may be app ied to either or both pole horns as may be ound preferable for the purpose desired. The generator in accordance with the invention is operated so that none of the iron parts of the magnetic circuit are saturated above theknee of the saturation curve between the voltage limits at short circuit and full load. Above full load, it is saturation which makes the top of the curve d1, a1, 61 (Fig. 6) bend over as shown from al to b1. The" term saturation employed in the above statement.

is intended to mean the general saturation of the magnetic circuit such as will ultimately occur if ma to a high enough va ue in the pole body, yoke, teeth, armature core etc. and does refer to purely local saturation of the pole tips or teeth under them. General saturation as distinct from local saturation is the cause of bending over of the curve above al (Fig. 6) but with general saturation alone the point b1 would usually be decidedly higher than the workin polnt a1. Saturation of the pole tip C ig. 1) has the effect of causing the curve to drop again after bending over so that by the introduction of an ap propriate amount of saturation at the poletip 0 the point b1 can be kept at a height not greater than that of a1 or even below al. A motor therefore working on load at the voltage represented by a1 would not increase in speed when the load is entirely removed.

For the pur ose of definition it is to be understood that y the term leading pole tip is meant the. pole tip which a point on the periphery of the armature first passes during rotation, and by the term trailin pole tip is meant the tip which a point on t e armature periphery last passes during rotation. Also y the use of the term iron parts of the machine, in the claims, is meant the iron parts of the generatonincluded in the complete magnetic path through the same, that is, the yoke, the pole (imtluding the shoe), the armature core and the armature teeth.

We claim 1. A constant current variable voltage electric generator of the aforesaid type, comprising an armature and a polar field system having graded polar air gaf ps, .the air gaps increasin in radial length rom the leading tips of t e poles to the trailing tips of the poles, the saturation of the iron parts of the machine being maintained below the knee of the saturation curve therefor for working conditions between the voltage limits at short circuit and full load, whereby a. drooping characteristic for the generator is obtained by the action of the armature cross ampere etization is raised turns acting across the graded air ugap of each ole to vary the value of the res tant polar ux.

2. A constant current variable voltage electric generator of the aforesaid type, comprising in combination, an armature, a polar field system with air gaps which taper, increasing in radial length from the leading to the trailing pole tips, a separately excited constant voltage magnetizing winding adapted to just overbalance the reverse armature magneto-motive-force on short circuit and a shunt magnetizing winding in shunt with said armature and acting in the same magnetic sense as said separately excited winding.

3. A constant current variable voltage electric generator of the aforesaid type, comprising in combination, an armature, a polar field system with air gaps which taper variably, increasing in length from the leading to the trailing pole tips, a separately excited constant but ad'ustable voltage magnetizing winding, and a shunt magnetizing winding in shunt with said armature and acting in the same magnetic sense as said separately excited winding.

4. A constant current variable voltage electric generator of the aforesaid type, comprising in combination, an armature, a polar field system with graded polar air gaps which increase in length from the leading to the trailing tips of the poles, at least one of the horns of each pole being of greater magnetic reluctance than the normal for the purpose of modifying the voltage-current characteristic, a separately excited constant voltage magnetizing winding, and a shunt magnetizing winding in shunt withsaid armature and acting in the same magnetic sense as said separately excited winding.

5. A constant current variable voltage electric generator of the aforesaid type, comprising in combination, an armature, a polar field system with air gaps which taper, increasing in length from the leading to the trailing pole tips, a separately excited constant voltage magnetizing winding, a shunt magnetizing winding in shunt with said armature so that the voltage across the shunt coils varies with the load, and a series magnetizing winding connected in series with said armature, all three windings acting in the same magnetic sense.

6. A constant current variable voltage electric generator of the aforesaid type, comprising in combination, an armature, a polar field system with air gaps which taper, increasing in length from the leading to the trailing pole tips, a separately excited constant voltage magnetizing windin a regulator in the circuit of said separate y excited winding whereby the value of the constant current supplied from the armature may be reduced or reversed when desired, and a shunt magnetizing winding in shunt with said armature and acting in the same magnetic sense as said separately excited winding.

7. A constant current variable voltage electric generator of the aforesaid type, comprising in combination an armature, a polar field system with air gaps which taper, increasing in length from the leading to the trailing pole tips, a separately excited constant voltage magnetizin lator in the circuit of sai separately excited winding whereby the value of the constant current supplied from the armature may be reduced or reversed when desired, a shunt magnetizing winding in shunt with said armature and acting in the same magnetic sense .as said separately excited winding, and a regulator in the circuit of said shunt winding for the purpose of varying the shape of the characteristic of the generator.

8. A constant current variable voltage electric generator of the aforesaid type, comprising in combination, an armature, a polar field system with air gaps which taper, increasing in length from the leading to the trailing pole tips, a separately excited constant voltage ma netizing winding, a shunt magnetizing win ing in shunt with said armature and acting in the same magnetic sense as said separately excited winding, interpoles over the brushes of the generator thereby enabling the characteristic of the generator to be varied by shifting the brushes within the imit of movement allowed by the interpole ace.

9. The combination with a constant current variable voltage electric generator of the aforesaid type comprising an armature and a polar field system having graded polar air gaps, the air gaps increasing in length from the leading tips of the poles to the trailing tips of the poles, a drooping characteristic for the generator being obtained by the action of the armature cross ampere turns acting across the graded air gap of each pole to vary the value of the resultant polar flux, of a main prime mover driving said generator, constant voltage exciting means for said generator, an electric motor excited by said constant voltage exciting means, and having its armature connected across the constant current mains from said generator, and a regulator connected in the constant voltage supply to the separately excited coils of the constant current generator.

10. In combination with a plurality of electric motors, a plurality of prime movers, one for each motor, and a constant current variable Voltage electric generator of the aforesaid type for each motor, adapted to supply current thereto and comprising an armature and a polar field system having graded polar air gaps, the air gaps increasing in len h from the leading tips of' the po es to e trailing tips of the poles, a

winding, a regudrooping characteristic for the ene-rator being obtamed by the action of t e armature cross ampere turns acting across the graded air gap of each pole to vary the value of the resultant polar ux, of constant voltage exciting means and means for connectin the motors in series or in parallel across at least one of the constant current generator output circuits for the purpose of reducing the number of prime movers re uired when the load on the motor is low or i one prime mover or generator should break down.

11. A constant current variable voltage electric generator of the aforesaid type, comprising in combination, an armature, a polar eld system with air gaps which taper, increasing in length from the leading to the trailing pole tips, a part of increased magnetic reluctance in at least one of the horns of each pole, a separately excited constant voltage magnetizing winding, a shunt magnetizing winding in shunt with said armature so that the voltage across the shunt coils varies with the load, and a series ma etizing winding connected in series wit said armature, all three windings acting in the same magnetic sense.

12. A constant current variable voltage electric generator of theaforesaid type, comprising an armature and a polar field system having graded polar air ga s, the air gaps increasing in radial length rom the leading tips of the poles to the trailing tips of the poles, the poles being constructed and the excitation being arranged, so that the saturation of the poles will be maintained below the knee of the saturation curve therefor throughout the working range of the ma- M chine from short circuit to full load.

13. A constant current variable ,voltage electric generator of the aforesaid type com prising in combination an armature, a polar field system with air ga' s which taper, increasing in radial length from the leading to the trailing tips, a separately excited constant voltage magnetizing winding, and a shunt magnetizing winding in shunt with said armature and acting in the same ma netic sense as said separately excited win ing, the parts being so proportioned that the open circuit voltage of the generator will be substantially the same as the full load voltage thereof, and the short circuit current will be substantially the same as the full load cur- -ent, while the output current will remain substantially constant with variation of voltage from the short circuit value (zero) up to the full load value.

In testimony whereof we afiix our signatures.

GILBERT AUSTIN. JAMES COLQUHOUN MACFARLANE. WILLIAM ALLEN MACFARLANF. 

